Patent Document 1 discloses a motor control device that controls a brushless motor, and a steering device for a vehicle in which the motor control device is used. According to the configuration disclosed in Patent Document 1, in power lines of a U phase, a V phase, and a W phase that connect a motor and a drive circuit (inverter), phase open switches, which cut off electric currents flowing to the phases, are respectively provided. From viewpoints of reduction in size, reduction in costs and the like, relay FETs formed of field effect transistors are used instead of conventional mechanical relay contacts in the phase open switches. The drive circuit is configured by connecting, with a pair of switching elements connected in series set as a basic unit (switching arm), three switching arms in parallel to correspond to motor coils of the respective phases.
In the FETs, parasitic diodes are present because of the structure of the FETs. Therefore, even if an open malfunction occurs in the relay FETs, energization in a first direction is allowed by the parasitic diodes. Therefore, in the configuration disclosed in Patent Document 1, a voltage is applied to only a specific phase in a state in which all the relay FETs respectively corresponding to the U phase, the V phase, and the W phase are turned on. Presence or absence of the open malfunction is determined according to whether a voltage based on the applied voltage is generated in the phases other than the specific phase. When the open malfunction is determined, only a switching element on a power supply side in the switching arm of the specific phase is turned on, whereby the voltage is applied to only the specific phase.
In this way, in the conventional configuration, the drive circuit is set to a special form different from a form during driving of the motor and a voltage is applied to the motor. Therefore, it is impossible to determine the open malfunction while driving the motor. That is, the open malfunction can be determined only under a limited situation during ignition-on or the like. Accordingly, when the open malfunction occurs during traveling of a vehicle, an abnormality of a control circuit cannot be quickly detected.
Note that Patent Document 2 discloses a method for detecting an energization failure that occurs when breaking of wire occurs in a power supply route between a motor and a drive circuit. Even with the method disclosed in Patent Document 2, the open malfunction of the control circuit cannot be detected.    Patent Document 1: Japanese Patent Application Laid-Open No. 2009-261067    Patent Document 2: Japanese Patent Application Laid-Open No. 2007-244028